The present invention relates to separation screens used with mechanically or pneumatically agitated tanks such as propeller or pachuca tanks.
Precious metals such as gold and silver are contained in an ore material when mined. Precious metal ores are commonly processed by grinding them into small particles, and placing them in a solution which dissolves the precious metals so that they can be separated from the ore. Typically, an aqueous and alkaline cyanide solution is used so that the metals form alkaline cyanide metal complexes in a water solution.
The most common method of separating the metal-containing solution from the ore simply involves filtering the solution from the ore, or separating the solution by countercurrent decantation in a series of thickeners. A relatively new method of recovering precious metals from ores is called the "carbon-in pulp" system and bypasses the liquid-solid separation step by mixing activated charcoal directly with the pulp of ore in cyanide solution. The charcoal or carbon remains in contact with the pulp for a sufficient time to adsorb the gold and silver and is then separated from the worthless residue, called tailings, by a screening process. The charcoal particles are larger than the ore particles which permits the screening step to be accomplished with ease.
A carbon-in-pulp system utilizes a series of mechanically or pneumatically agitated tanks in series, usually 4 to 6. Each tank generally contains a different amount of charcoal, the first tank having the most charcoal and the last tank having the least. Pulp, i.e., the finely ground ore and the alkaline cyanide metal complex solution, is introduced into the first tank. The pulp is agitated with the charcoal in the tank by imparting an upward rolling motion to the mixture outwardly from the center of the tank to its periphery and then down the sides of the tank. The charcoal adsorbs the cyanide metal complex as the pulp is agitated. The pulp is sequentially passed through the series of pneumatically or mechanically agitated tanks so that most of the cyanide metal complex is adsorbed by the charcoal.
Passage of the pulp material through the series of agitated tanks is controlled in many systems by vibrating screens mounted over the tanks, and in other systems by stationary vertical peripheral screens attached to the outer top edges of the tanks. The charcoal particles are sufficiently large so that they do not pass through the screens, and are retained in each tank. The ore is sufficiently finely ground so that the pulp passes through the screens and on to the next downstream tank in series. After passing through the series of tanks, the processed pulp tailings are discarded.
The charcoal containing the adsorbed cyanide metal complex is sequenced through the tanks in reverse order from the ore pulp, new charcoal being added at the final tank. After the charcoal has passed completely through the system, it has become "loaded" with the adsorbed metal complex. In a gold refining system the charcoal can adsorb up to 400 or 500 oz. of gold per ton of charcoal. The leaded charcoal is then chemically processed to remove most of the metal, and after such chemical processing, the "stripped" carbon still contains about 5 oz. of gold per ton of charcoal in a typical gold refining system. The stripped charcoal is then reactivated by heating in an absence of air and returned to the carbon-in-pulp system.
The carbon-in-pulp processing system described above is quite efficient in recovering the precious metals. However, the system relies on the assumption that the charcoal containing the adsorbed metal does not pass through the vibrating or vertical peripheral screens. If a charcoal particle is sufficiently small to pass through the screens, it will be passed completely through all the tanks and lost in the tailings.
The charcoal particles used in the carbon-in-pulp system are carefully sized before entering so that they will be too large to pass through the screens. However, during the operation of the system charcoal particles can become abraded or broken. If the charcoal particles are abraded or broken so as to be sufficiently small to pass through the screens, the precious metal on such charcoal particles is lost in the tailings.
A principal source of abrasion and breakage of the charcoal particles is the abrasion of the charcoal against the screens. Because of the rapid abrasion of carbon by vibrating screens, fixed peripheral screens have been mounted on the upper rim of the agitated tanks. Such screens now in use are vertical but because the flow of pulp holds the larger carbon particles against the screens, thus "blinding" them, air jets are typically used to remove the carbon particles and keep the screen open to the flow of pulp.
The air jets cause intense agitation of the particles against the screens, abrading the edges of the charcoal particles so that eventually the particles may be rendered sufficiently small to pass through the screens. The loss of charcoal from this and other sources of abrasion, even in the best designed systems, is typically about 0.1 lb. or more of charcoal per ton or ore. Some systems use as much as 0.5 lb. charcoal per ton of ore treated. For a typical gold processing system, this can result in the loss of thousands of dollars per day in adsorbed gold discarded with the tailings.